The present invention relates to an exhaust control apparatus for an engine, particularly an exhaust control apparatus for an engine which has a turbocharger and an EGR device.
Conventionally, in order to improve emission performance of engines, exhaust gas recirculation (EGR) devices for recirculating part of exhaust gas within an exhaust passage to an intake passage are known. To achieve such an improvement, arts using two EGR devices including a high-pressure EGR device and a low-pressure EGR device are also known. The high-pressure EGR device recirculates exhaust gas from a position of an exhaust passage upstream of a turbine of a turbocharger to a position of an intake passage downstream of a compressor of the turbocharger, and the low-pressure EGR device recirculates the exhaust gas from a position of the exhaust passage downstream of the turbine of the turbocharger to a position of the intake passage upstream of the compressor of the turbocharger. For example, JP2007-303380A discloses such an art.
Generally, when using such two EGR devices, the high-pressure EGR device is operated within an engine operating range where the engine load and speed are low, and the low-pressure EGR device is operated within an engine operating range where the engine load and speed are higher than the range where the high-pressure EGR device is operated. Note that the engine operating range where the high-pressure EGR device is operated is suitably referred to as the “high-pressure EGR range,” and the engine operating range where the low-pressure EGR device is operated is suitably referred to as the “low-pressure EGR range.”
As turbochargers, variable geometry turbochargers (VGTs) provided with a plurality of movable flaps (i.e., movable vanes or nozzle vanes) around a turbine thereof are conventionally known. JPH09-280119A discloses a system using such a turbocharger and an EGR device. The system mechanically fixes flap openings to a predetermined opening when the exhaust gas is recirculated by the EGR device.
Incidentally, within the high-pressure EGR range, the flap openings of the turbocharger are preferably controlled while giving the highest priority to the controllability of the exhaust gas recirculation performed by the high-pressure EGR device, so as to improve ignitability and emission performance of the engine. Specifically, within the high-pressure EGR range, it is not preferable for an exhaust gas pressure to be changed by the operation of the turbocharger.
On the other hand, within the low-pressure EGR range, the flap openings of the turbocharger are preferably controlled to cause the turbocharger to operate to a certain extent while securing the controllability of the exhaust gas recirculation performed by the low-pressure EGR device. Specifically, since the length of the path for the exhaust gas to be recirculated by the low-pressure EGR device is long, the flap openings are preferably controlled while taking into consideration that the responsiveness of the exhaust gas recirculation by the low-pressure EGR device is low.
Thus, the flap openings of the turbocharger are preferably controlled according to the high-pressure EGR range and the low-pressure EGR range individually. Such control of the flap openings according to the individual EGR ranges is disclosed in neither JP2007-303380A nor JPH09-280119A.